Inhibitory activity toward beta-glucosidase was detected in extracts of the lichen, umbilicaria esculenta. The extract showed strong inhibition of disaccharide hydrolytic enzymes of mold and mammalian origin, but weak or no inhibition of polysaccharide hydrolytic enzymes except glucoamylase and laminarinase. The inhibitor in the extract was very stable, retaining more than 95% of its activity when treated with heat, acid, alkali, and some hydrolytic enzymes. Purified inhibitor was identified as 1-deoxynojirimycin (1,5-dideoxy-1,5-immino-D-glucitol) by NMR spectrometry, which was known to be produced by Streptomyces sp. and the plant Morus sp. Extracts from Parmelia austrosinensis, Parmelia praesorediosa, and an unidentified lichen species, showed glucosidase inhibitory activities similar to U. esculenta.